1. FIELD OF THE INVENTION
The present invention relates to an optical current transformer using combination of a magnetic core and a magnetic detecting device such as optical cell.
2. DESCRIPTION OF THE PRIOR ART
A conventional optical current transformer, for example, shown in Published Unexamined Japanese Patent Application Sho 59-198359 is described referring to FIG. 5 and 6.
FIG. 5 is a sectional front view showing three-phase gas insulation bus bars using a conventional optical current transformer 5. In FIG. 5, three bus bars 2, 3 and 4 are positioned in a tube-shaped housing 1 which is made of metal. The conventional optical current transformers 5 are provided around the bus bars 2, 3 and 4.
Details of the conventional optical current transformer 5 is shown in FIG. 6. In FIG. 6, a C-letter-shaped iron core 6 with a gap part 7 which is filled by plastics or glass forms a ring-shaped magnetic path which interlinks the bus bar 2. A Faraday cell 8, which is one of magnetooptical cells, is provided in the gap part 7 of the core 6. Optical fibers 9a and 9b are connected to the Faraday cell 8. The optical fiber 9a is used for supplying a light beam to the Faraday cell 8 and the other optical fiber 9b is used for guiding the light from the Faraday cell 8 to a photoelectrical signal processing apparatus 15 transforming the light beam to an electric signal. A winding 10 is provided in the gap part 7 and in the vicinity of the gap part 7 of the core 6 for canceling external magnetic field. Another winding 11 is provided in a part of the core 6 symmetrically to the winding 10 against the bus bar for detecting the external magnetic field. Number of turns of the windings 10 and 11 are selected equal to each other. And the windings 10 and 11 are connected in a manner that voltages induced in the windings 10 and 11 by a main magnetic flux .phi.1 induced by the current in the bus bar 2 and circulating in the core 6 are canceled each other, since plural voltage sources of equal electromotive forces are connected commonly without any load to it.
Operation of the conventional optical current transformer 5 is described.
When a current which is to be detected flows in the bus bar 2, the main magnetic flux .phi.1 in proportion to the intensity of the current circulates in the core 6. The main magnetic flux .phi.1 also flows in the gap part 7. Hereupon, when a light beam is supplied to the Faraday cell 8 via the optical fiber 9a, the light beam is modulated in proportion to the intensity of a magnetic field due to the main magnetic flux .phi.1 in the Faraday cell 8. The modulated light beam is outputted from the Faraday cell 8 via the optical fiber 9b and is photoelectrically transformed by the signal processing apparatus 15. As a result, the intensity of the current flowing in the bus bar 2 is detected.
The windings 10 and 11 are used for restraining undersirable influences caused by external magnetic flux .phi.m (shown in FIG. 6) generated by currents flowing in the other bus bars 3 and 4 (shown in FIG. 5). As shown in FIG. 6, assuming the external magnetic flux .phi.m is applied to the windings 10 and 11 even, then, the voltages induced in the windings 10 and 11 by the external magnetic flux .phi.m are not canceled each other, and the winding 10 generates a magnetic flux which has opposite direction to that of the external magnetic flux .phi.m. As a result, the part of the external magnetic flux .phi.m which is in the gap part 7 is canceled by the magnetic flux generated by the winding 10. That is, by the existences of the windings 10 and 11 on symmetrically opposite parts on the circular iron core 6, undersirable influencing to the main magnetic flux .phi.1 in the circular core 6 by the external magnetic flux .phi.m is reduced by the canceling. And thereby the inherent current detection of the conventional optical current transformer 5 is not injured.
However, in the above-mentioned conventional optical current transformer 5, the number of the windings 10 and 11 are only two, and the widths thereof are not large enough, thereby leaving considerable part of the circular core 6 un-wound. Hence, there is a problem that the influences caused by the external magnetic flux .phi.m are not sufficiently or ideally removed, because the windings 10 and 11 for removing the influence caused by the external magnetic flux .phi.m are only provided in the gap part 7 and in the vicinity of the gap part 7, as well as the parts symmetrically opposite thereto, respectively.
However, in the actual housing 1, the external magnetic flux .phi.m is not ideally uniform nor straight. And in such case, the protection function by the windings 10 and 11 is not perfect, and the influences in the parts of the core 6 which is other than the gap part 7 by the external magnetic flux .phi.m is induced. Therefore, magnetic permeabilities of various parts of the core 6 becomes uneven because of unevenness of the intensities and directions of the external magnetic flux .phi.m, and hence, overall magnetic reluctance of the core 6 is varied from designed value, depending on the path and unevenness of the external magnetic flux .phi.m. As a result, an error component of output current caused by the external magnetic flux .phi.m is reflected in the measured current, and the magnetic flux detected by the faraday cell 8 contains error.